In semiconductor devices, polysilicon films are widely used to form the gate electrodes of transistors, etc. In a deep sub-micron area of a highly-integrated semiconductor device, in particular, a highly reliable technique is required, for example, to bury an electrode in a contact hole, and polysilicon films are indispensable to such a technique. The polysilicon films are generally doped with impurities.
In light of the above, further development of methods for doping polysilicon films with impurities is being demanded.
Conventional methods for forming a polysilicon film doped with e.g. phosphorus (P) include a method for implanting P into a polysilicon film by ion implantation and annealing the film, and a method for forming a P.sub.2 O.sub.5 film on a polysilicon film with the use of POCl.sub.3 gas and then subjecting the resultant to a diffusion treatment. Moreover, the "in-situ method" is known as a method using HOT-WALL-type vacuum CVD.
The method for injecting phosphorus by ion injection, however, has the drawback that the crystal of the polysilicon film may be broken by the shock of ion implantation. Further, the method using POCl.sub.3 gas has the drawback that the concentration of diffused impurity is not uniform in the depth direction, and a process for removing the P.sub.2 O.sub.5 surface film is necessary, which complicates the manufacture of the device.
On the other hand, the in-situ method does not have the above-described drawbacks, and has the advantage that the concentration of phosphorus in a thin film can be controlled by adjusting the flow of a dopant gas. Therefore, the in-situ method is widely employed as an effective method for forming a polysilicon film.
Referring to FIG. 1, the in-situ method will be explained. FIG. 1 is a cross sectional view, showing a film-forming apparatus employed in effecting the in-situ method. A heating furnace 1 has a heater 3, and a reaction tube 2 of a double-tube structure is located in the furnace. A manifold 5 equipped with film-forming gas inlet pipe 6, a dopant gas inlet pipe 7 and an exhaustion pipe 8 is located on the lower end of the reaction tube 2. In the film-forming apparatus, a wafer boat 4 in which a plurality of wafers w, e.g. 100 wafers are mounted, is transferred into the reaction tube 2 from its lower opening, using a boat elevator 9. Thus, the wafers W are loaded in the tube. Before loading the wafers W, the reaction tube 2 is heated to a predetermined temperature. Then, gas exhaustion is performed so as to reduce the pressure in the reaction tube 2 to a predetermined vacuum value. At the same time, a film-forming gas as a mixture of monosilane (SiH.sub.4) gas or disilane (Si.sub.2 H.sub.6) gas and a diluent gas such as helium (He) gas is supplied into the reaction tube 2 through a film-forming gas introduction pipe 6, while a dopant gas as a mixture of phosphin (PH.sub.3) gas and a diluent gas such as helium (He) gas is supplied into the reaction tube 2 through a dopant gas introduction pipe 7. Thus, a thin film is formed on the surface of each wafer W as a result of gas phase reaction.
Recently, the allowable range of the quality or thickness of such a film has been narrowed in accordance with the advance in refining a device or enhancing the reliability of the same. In the in-situ method, too, higher uniformity has been required in the in-plane film quality or thickness of each wafer, or in film quality or thickness between wafers arranged in the longitudinal direction of the reaction tube. To meet the requirement, control of the reaction temperature by providing a heater 3 of a division type around the reaction tube 2 or varying the temperature of the reaction tube 2 in its longitudinal direction has been performed so as to enhance the uniformity in the in-plane film quality or thickness of each wafer.
However, since in the above-described methods, phosphorus is supplied into the reaction tube 2 through the dopant gas introduction pipe 7 located at a lower end portion of the tube 2, the gas is consumed on wafers while it flows upward. As a result, the concentration of phosphorus contained in the gas becomes lower as the gas reaches a higher portion of the tube 2, and the uniformity in the concentration of phosphorus deposited on wafers is degraded, inevitably reducing the yield of products.
To solve this problem, the present inventors examined whether or not an L-shaped dopant gas introduction pipe is effective which is bent upward so that its upper portion can be located at an upper portion of the wafer boat, and has a plurality of gas outlets. In this case, however, there is a pressure difference between gas discharged through a gas outlet formed in a lower portion of the gas introduction pipe, and gas discharged through a gas outlet formed in an upper portion of the same. In other words, gas flows at different speeds in different portions of the tube. To avoid this, it was found necessary to appropriately vary the diameter of each gas outlet or the distance between each pair of adjacent gas outlets must be varied appropriately. However, it is very difficult to do so. In addition, after the film-forming treatment, the gas introduction pipes are cleaned, using hydrofluoric acid which can dissolve a metal constituting the pipes. Therefore, the diameter, etc., of each gas outlet may well be varied.